This invention relates to the testing of the integrity of in vivo bones to determine either the possible existence of a fracture or the progress of healing of a fracture. More particularly, the invention relates to a system for testing the integrity of such bones noninvasively by inducing a mechanical vibration in the bone, detecting the vibration by means of an electrical vibration transducer, producing an electrical signal representative of the vibration, and analyzing the vibration from the electrical signal.
In current medical practice, a number of techniques are used to detect bone fractures and to evaluate the healing progress of a fracture. Fracture detection techniques include examination of X-rays and radio isotope scanning. Fracture healing evaluation techniques include observation of stability of the bone, knowledge of the patient's history, and examination of X-rays of the fracture. All of these measures require substantial experience and involve subjective evaluation of the state of the bone. As a result, small or partial fractures such as stress fractures are sometimes undetected, while some non-unions are identified too late in the healing evaluation process for timely surgical intervention. Accordingly, a more objective measure of the existence of a fracture and of the healing state of a fractured bone would be a useful clinical tool.
A number of biomedical researchers have explored various measures of the mechanical properties of bone. Prior researchers modeled bones as beams, evaluating them under a variety of boundary conditions and loading and predicting a hyperbolic relationship between rigidity of a fracture callous and the percentage of healing. Others developed and tested conceptual models of the human ulna to be used in the prediction of ulnar resonant frequency. Markey and Jurist in 1974 suggested that the natural frequency of the tibia could be used as an index of fracture healing. Lewis in 1975 presented a dynamic model of a healing fractured long bone and asserted that the peak response of an accelerometer attached to a healing bone was a measure of its stiffness. Collier et al. in 1982 presented theoretical models and experimental verification of the mechanical resonances of the human tibia in vitro. Others found a correlation between the transmission coefficient of elastic waves traveling through the fracture site and the degree of union. Sonstegard and Matthews in 1976 reported on the impulsive time responses of fractured bones, while Hirayama and Sekiguchi in 1979 used the frequency response and the time waveform of the impulse response. White et al. in 1976 reported that a normally healing fracture shows a sudden increase in stiffness as the fracture callous changes from soft tissue to bone. However, all of the foregoing mechanical techniques were invasive, or time-consuming and complicated, or all of these. They did not suggest how to obtain reliable results quickly by noninvasive testing of in vivo bones.
The present inventor, in conjunction with others, has previously suggested use of the spectral content of the pulse response of in vivo long bones in the evaluation of fracture healing, and has presented results of such spectral content using a noninvasive technique and a microprocessor-based signal analyzer to obtain the Fourier transform of the pulse response from which the predominant resonant frequency can be determined, as set forth in articles published in 1979, 1982 and 1983. These articles point out that a fractured bone will have a different spectral content of the pulse response than a healthy contralateral bone of the same type, and that the spectral content of the fractured bone should approach that of the healthy bone as healing progresses. The resonant frequency indicated by the predominant peak of the Fourier transform is the most easily recognizable feature of the spectral content for purposes of comparing the fractured bone with the healthy bone.
If the foregoing spectral content comparison technique could yield consistent results, there would be no need for improving upon it. However, numerous factors combine to make the results inconsistent. For example, although the resonant frequency is proportional to stiffness of the bone which, in turn, is indicative of bone integrity and progress of healing, the resonant frequency is also inversely proportional to such factors as mass and damping. Accordingly, factors other than stiffness which can affect the resonant frequency exhibited by the pulse response of an in vivo bone include a large amount of callous formation from healing which adds mass to the system, and synostosis which couples another bone into the system (such as coupling the fibula to a healing tibia). Moreover, the amount of muscle and other soft tissue surrounding a bone affects both the mass and the damping of the system, thereby also affecting its resonant frequency in a manner unrelated to the stiffness of the bone. Although the latter variables could theoretically be compensated for by comparison of the spectral content of the pulse response of the tested bone to that of a healthy corresponding contralateral bone, other variables such as possible unavailability of an intact contralateral bone, different testing techniques by different operators, and different muscle tension of the respective limbs during testing, make the magnitude of the resonant frequency alone too unreliable an indicator to yield consistently accurate results.